This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The objective of this proposal is to investigate the mechanisms by which aging affects physical functioning and aerobic capacity in HIV-infected adults, and to determine if aerobic exercise training improves functional and cardiovascular performance. Results will be used to design and test specific exercise interventions for older adults infected with HIV. Our long-term objective is to devise effective prevention and treatment strategies that will improve and preserve function in this rapidly growing older patient population. Specific Aim: To determine the effect of 20 week aerobic exercise training (16 weeks endurance training plus 4 weeks short-sprint interval training) on skeletal muscle oxidative function and aerobic capacity in older HIV adults. We will conduct a randomized trial of 60 HIV subjects = 55 years. The following parameters will be assessed at baseline,16 weeks, and 20 weeks in the treatment group (N=30) and control group (N=30): a) aerobic capacity (VO2peak);b) functional performance;c) skeletal muscle oxidative function (Cytochrome C oxidase (COX) and succinate dehydrogenase (SDH);d) skeletal muscle mitochondrial morphology (electron microscopy) and mtDNA (mt/nuclear DNA ratio by PCR);e) lean body mass (DEXA;CT scan);f) skeletal muscle histology (HIV-related myopathy). Hypothesis 1. Skeletal muscle oxidative dysfunction is a mechanistic factor underlying reduced aerobic capacity and functional limitations in older HIV adults. Hypothesis 2. Aerobic exercise training will improve muscle oxidative dysfunction, and the associated functional limitations and reduced aerobic capacity in older HIV subjects compared to sedentary older HIV controls;however, improvements may be limited by the extent of sarcopenia and HIV-related myopathy. Design/procedure: This randomized trial compares the effect of 20-weeks aerobic exercise training versus stretching control in HIV-infected adults = 55 years. Outcomes include physiologic, functional, muscular, metabolic, and body composition measures which will be assessed at baseline, 16 weeks, and 20 weeks. All subjects will be HIV-infected and on stable highly active antiretroviral therapy (HAART). The majority of the volunteers will be recruited from the ID Clinic at the Baltimore VA Medical Center (VAMC)(census 620) and the University of Maryland adult HIV clinic, Evelyn Jordan Center (EJC) (census 1,800), and the community. We will use the same recruitment strategies and eligibility criteria recently employed to enroll 63 older HIV-infected adults into a cross-sectional exercise study (GET Fit, H-25827). Sixty eligible participants will be will be stratified by gender and AIDS history into one of four strata: (1) Male, history of AIDS;(2) Male, no history of AIDS;(3) Female, history of AIDS;(4) Female, no history of AIDS. Available subjects in each stratum will be assigned at random 1:1 to either the experimental group or the stretching control group, using the closed envelope method. Targets per training phase for the aerobic training group are expressed as percent heart rate reserve (HRR). The control group will complete 20 weeks of a supervised center-based stretching program. The study phases are listed here: Phase 1- Screening testing and randomization. Phase 2- Baseline outcomes testing. Phase 3- aerobic base training (4 weeks), goal end is 30 minutes at 60-70% HRR. Phase 4- endurance training (12 weeks), goal is 45 minutes at 70-85% HRR Phase 5- follow-up testing #1 Phase 6- short-term sprint interval training (4 weeks), goal is 6 sets of 30 second intervals at 90+% HRR with 4-6 minutes recovery. Phase 7- follow-up testing #2